Sweating

Sweating (also called perspiration) is the production of fluid by the sweat glands and its evaporation from the skin. It is the principal way in which the human body loses heat when the surrounding air approaches or exceeds skin temperature, and it is therefore central to the physiology of sauna bathing, where bathers sit in air heated well above the temperature of the body.

Thermoregulatory role

The normal deep-body temperature of a resting human is about 36.5 to 37.5 °C, and Thermoregulation keeps it within narrow limits. Heat is normally lost by radiation and convection, but when the air temperature rises above about 35 °C these routes lose their effectiveness and the skin becomes warmer than its surroundings. Sweating then becomes the most important channel of heat loss: each litre of water evaporated from the skin removes roughly 2.3 megajoules (about 550 kilocalories) of heat. Evaporation is driven by the heat of the skin itself, so its cooling effect depends on dry air moving over the skin rather than on the temperature of the room.

Sweat is produced by eccrine glands distributed over nearly the whole body surface; humans have on the order of two to four million of them. Their activity is controlled by the sympathetic nervous system, and a rise in Core body temperature of only a fraction of a degree is enough to trigger secretion. Sweating is accompanied by Vasodilation of the skin vessels, which raises Skin temperature and delivers warm blood to the surface so that heat can be transferred to evaporating sweat. During heavy exposure to heat the skin can receive several litres of blood per minute, and Blood circulation and heart rate rise accordingly.

Sweating in the sauna

In the hot room of a sauna, at air temperatures commonly between 70 and 100 °C, the body gains heat by radiation and convection instead of losing it, so evaporation of sweat is virtually the only escape route for metabolic heat. For a bather sitting still, visible sweating usually begins within a few minutes, even in a bather who is not otherwise warm, because the skin is heated directly. Skin temperature climbs toward about 40 °C and core temperature typically rises one to two degrees over the course of a session; both gradually return to normal afterwards. The average fluid loss during one sauna bath is of the order of half a kilogram, mostly as sweat, with a typical range from about 0.3 to 1.0 kg depending on the heat, the humidity and the length of the stay.

The humidity of the sauna matters for two reasons. First, water vapour added by throwing water on the stones (löyly) reduces the rate at which sweat can evaporate, so sweating tends to be more profuse without producing proportionally more cooling. Second, sweat that runs off the skin without evaporating carries heat away inefficiently and serves mainly as a sign that the body is working hard to dissipate heat. Under dry-room conditions most sweat evaporates, whereas in a very humid cabin it may drip; the practical consequence is identical fluid loss either way. The cardiovascular work associated with hot-room exposure is discussed under Cardiovascular effects of sauna; regular exposure is also one way to bring about Heat acclimation, which increases sweat rate, lowers the sodium content of sweat and reduces the sweating threshold.

Composition of sweat

Sweat consists mainly of water, normally more than 99 per cent, with sodium and chloride as the major dissolved salts. It also contains potassium, calcium, magnesium, urea, lactate, ammonia and small amounts of other substances. The concentrations vary with the rate of sweating, diet, hormone status and, above all, with acclimatisation: sweat from a heat-acclimatised person is more dilute, so that less sodium is lost per litre of fluid. Because sweat is dilute relative to blood plasma, profuse sweating causes a proportionally larger loss of water than of electrolytes. Measurement of the composition of collected sweat is used in research and in the diagnosis of some conditions; see Sweat analysis.

Fluid and electrolyte losses

A single session in a hot sauna can remove well over a litre of fluid when the bather stays long, is acclimatised or moves about; the fluid must come from the body's water stores. Dehydration is the main preventable risk of this fluid turnover, and the sensation of thirst is a late indicator of water deficit. Drinking water before, during and after the session is the usual counter-measure, and in a heated part of a facility this is a matter of routine (see Hydration during sauna). Electrolyte balance is rarely disturbed by ordinary recreational sauna bathing, because the quantities of sodium lost in a short session are modest and are replenished by a normal diet; sustained losses become more relevant after very long or repeated sessions, and both fluid and salts are then restored with food and drink (see Eating after sauna and Food and drink in a wellness facility). Signs such as dizziness, headache or nausea during or after a sauna should be treated with caution, since they may indicate incipient heat illness rather than simple sweaty discomfort; the precautions are summarised under Sauna safety.

Sweating, detoxification and weight loss

Because sweat removes small quantities of metabolic waste such as urea, sweating is sometimes presented as a way of "ridding the body of toxins". The amounts of heavy metals and other pollutants that can be excreted in sweat are small, and the kidneys and liver are the organs that normally remove waste products from the body. There is no good evidence that deliberate sweating improves the removal of pollutants beyond a trivial amount, and claims that a sauna "detoxifies" the body are not supported by the physiological literature (see Detoxification claims about sauna).

Likewise, the fall in weight that follows a sauna session is almost entirely water lost in sweat, which is restored when the bather drinks and eats. It is not a reduction in fat mass, and the scale reading is regained within a day; see Weight loss and sauna and Calories burned in a sauna.

Sweat, skin and hygiene

Sweat keeps the outer layer of the skin, the stratum corneum, hydrated, which is one reason skin complaints such as eczema and psoriasis are discussed in connection with bathing. At the same time the warm, damp surface of a sweating body is a good medium for skin microorganisms, so showering before and after the sauna, drying carefully and not sitting on unwashed benches are standard hygienic precautions (see Skin and sauna and Sauna hygiene). For most people, profuse sweating in the sauna is a normal and uneventful part of the bath; the physiology and the practical management of fluid loss that surround it are well documented in the specialist literature.

References

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